1. Field of Invention
This invention relates to a composition and a process for forming sintered, molded articles having improved dimensional stability.
2. Description of the Related Art
Injection molding is a well known process for forming thermoplastic molded articles, such as plastic bottles or containers. Other types of useful injection molded articles are formed from metal powder (metal injection molding, or "MIM") or ceramic powder (ceramic injection molding, or "CIM"), rather than a thermoplastic material. Powder injection molding generally involves injecting a moldable fluid composition, comprising a combination of a ceramic or metal powder, a gel forming binder and a solvent into a mold of a predetermined shape under conditions sufficient to form a shaped article, referred to as a "green body." After forming a ceramic or metal green body, it is often desirable to sinter the article to remove any residual solvent and density the article. However, sintering frequently causes undesirable cracks or distortions in the article.
In order to remedy this problem, it has been suggested to improve the mechanical strength of the fluid composition. This is accomplished through increasing its solid content. In particular, when using a powder containing composition, increasing the powder content will increase the mechanical strength of the composition and decrease the probability that a shaped article will crack during sintering. To the contrary, a composition having a lower powder content will cause cracking and shrinkage in molded articles formed therefrom during sintering.
To increase the amount of powder a composition can retain, it may be necessary to augment the applicable binder with at least one viscosity reducing additive. The binder generally comprises a water soluble component or a polysaccharide that combines with the powder to form a flowable gel. Optimally, the additive will serve to increase the powder holding capacity of the binder, decrease the porosity of a molded article, and result in sintered articles having superior mechanical and physical properties.
Compositions of the prior art have generally been unable to form green bodies having sufficiently high density and dimensional stability. Various attempts have been made using different additives to increase the solid holding capacity, and thus the mechanical strength, of a ceramic or metal composition. For example, U.S. Pat. No. 5,746,957 teaches a process for forming ceramic and/or metal articles comprising a ceramic and/or metal powder, a polysaccharide binder, a solvent and a gel strength enhancing agent comprising a borate compound. The borate compound is incorporated to increase the quantity of solid powder material that the composition can retain, thus increasing the density of the product. U.S. Pat. No. 5,950,063 teaches a powder injection molding process using a composition comprising a powder and a binder incorporating additives such as coupling agents, antioxidants and surfactants. Also, U.S. Pat. No. 4,734,237 provides a process for injection molding metallic or ceramic articles from a mixture comprising a metal or ceramic powder, a gel-forming material having specific desirable properties and a gel-forming material solvent. The specific gel-forming material allows for increased solid retention in the moldable composition.